These are the simplest forms of life. But all the higher forms of life, and we may say, roughly speaking, any form of life that the unaided eye can see, is made up not of one cell but of many cells. A human being, for example, is made up of uncounted millions of cells; and millions of cells go to the formation of a worm, a fish, a gnat, or indeed to the formation of the simplest well-known animal that the ordinary person could name. Similarly millions of cells go to the formation of a leaf or a twig. These higher forms of life are called "multicellular organisms," because they have many cells, and most often many different kinds of cells. For instance, in the body of a man there are different kinds of cells to form the skin, or the lining of the mouth, or the substance of the eyes, or the red or white corpuscles of the blood, or the grey matter of the brain, or the roots of the hair--to name only a few. Thus we see how complicated the structure of animals has become since life first made its appearance on the earth. The cells joined themselves to form tissues; and the tissues joined themselves to form organs; and these things had to happen before anything like a complete animal of the higher type, or even a complete vegetable, made its appearance. Suppose by some great cataclysm, not so great as that which we have imagined in an earlier chapter, but still world-wide in its effects, the whole world should once again be swept by a great outbreak of lava and molten rocks, which of all the living things would leave traces of its existence? Perhaps a few of the animals with great bones, or the great trees, might leave an impress of themselves in the depths of the overwhelming rocks, just as we can stamp the impress of the skin of our finger-tips on hot sealing-wax; but it is fairly evident that all the soft-tissued animals and vegetables would disappear entirely and leave not a trace behind--certainly no trace that anybody could recognise many millions of years afterwards. It is still more certain that the simplest forms of life, "the unicellular organisms," would leave no trace at all. We know that side by side with the complicated organisms that we can see the simplest organisms exist now; and must have existed at the beginning of life. Yet when we examine the records of living things in the rocks which were formed in the youth of the world, and go back right to the earliest of these forms of life that have ever been discovered, we find that such specimens are all of the rather higher (if not of the highest forms of life). From that we infer that life must have existed many ages before the period of such remains, though, as we should have expected, all examples of the earlier forms have disappeared.